The great majority of oil production, especially vegetable oil production, includes oil extraction and then oil refining. Hexane is a solvent commonly used to dissolve the oil and form less viscous solution called miscella. The crude oil or miscella must then be subjected to additional treatments, called refining, to remove various materials. These materials include free fatty acids, phospholipids, color and flavor components, etc. Biodiesel after synthesis includes methanol, catalyst, water, soaps, and glycerol, which should be also removed. Presence of impurities should not be above ACT and other standards.
A wide variety of oils, primarily for food, and biofuel uses, in addition to crude, include organic acids, such as free fatty and naphthenic acids. These components must be removed, in the case of food oils to minimize rancid taste and in the case of biofuels and crude oils to reduce acid corrosion of metal parts. One of the important parameters to characterize oil quality is total acid number (TAN), and its decrease leads to an increase of market price of the oil. It is defined as the number of milligrams of potassium hydroxide required to neutralize the acids in a sample (mg KOH/mg sample). TAN is important also for crude petroleum oils. In this case the potassium, sodium and calcium hydroxides or monoethanolamine are used to neutralize the oil, but naphthenic acids, which are the main component leading to the high TAN, remain in the crude together with metal ions, and their separation and removal is complicated. Related to TAN characteristic is an acid value, which is based on pH in aqueous solution preequilibrated with oil.
The free fatty acids are removed from the oil by the process known as chemical or alkali refining. In this process, the oil is usually mixed with alkaline aqueous solutions of sodium or potassium hydroxides, washed, and then separated from alkaline solution. Nonhydratable calcium and magnesium salts of phospholipids are also removed along with fatty acids. An aqueous phase formed after separation from the neutralized oil is called a soapstock. Hydratable phospholipids are usually removed by a process called degumming, which includes treatment with aqueous or acidic solutions.
Chemical oil refining is an expensive process, requiring a large investment in equipment, but it also leads to partial saponification reaction of oils. A significant quantity of the oil is captured by the soaps and stable water-in-oil emulsions formed in the process. Conventional oil refining also involves bleaching with clays, silica or other adsorbents and deodorization at high temperature and vacuum. This treatment of edible oils leads to loss of valuable components including antioxidants.
Currently preferred method of accomplishing deacidification is to create an emulsion of the oil and an aqueous solution of an alkali such as sodium hydroxide (NaOH). These liquids are mixed for a prolonged period before separating the two non-miscible fluids, commonly by means of mixer-settler and/or centrifugation. Neutralization has to be fast, and to reach this it is necessary to have large surface area of water emulsion in oil. This leads to small size of the aqueous droplets, which are additionally stabilized by surface active fatty acids and are difficult to remove.
The treated oil can be subjected to electrofiltration by passing through a solid particles bed with an imposed dc electric field having a gradient of at least 20 kV/inch. Other methods like electric desalting and distillation have also been used. All these methods are each technically, temporally or monetarily inefficient.
Refining of oils can be conducted using pressure-driven membrane processes. Examples of relevant prior art are:
U.S. Pat. Nos. 4,062,882 and 4,533,501 describe membrane filtration under pressure to separate phospholipids from oils dissolved in nonpolar solvents, including hydrocarbons. Phosphatides form micelles in these solvents, and are retained while oils are passing through the membrane. Separation selectivity is based on different size of micelles and oil molecules.
U.S. Pat. No. 5,545,329 describes a module with many flat parallel polyimide membranes and pressure-driven separation.
U.S. Patent application US 2005/0118313 describes a microfiltration membrane-based method to separate lecithin from the miscella. The pore size in a polymer membrane was in the range from 0.1 to 2 micron.
International patent WO 2008/002154 A2 describes a process for reducing the free fatty acid content of natural oils by direct contacting the crude natural oil with an immiscible solvent to produce depleted in free fatty acids oil and a free fatty acid rich solvent phase. This last phase was further processed with pressure-driven membrane treatment to separate the free fatty acids from impurities of glycerides. The method still needs physical mixing and then pressure-driven separation, it needs large volumes of this solvent and does not allow an easy separation of fatty acids from it.
Ind. Eng. Chem. Res. 1992, 31, 581 describes extraction of fatty acids from oil using ultrafiltration hydrophilic cellulose-based hollow fiber membrane module. Fatty acids were extracted from oil to 1,2-butanediol, and then they were removed by addition of water and demixing of the acceptor phase. Because of low interfacial tension to keep both donor and acceptor phases separated membrane pore size was 3.5 nm.
Another type of membrane-based process is based on hollow fiber membrane contactors, which have found many applications for debubbling and gas removal from water and organic solutions. See for example, U.S. Pat. No. 6,402,818 B1. Selective separation is possible here not because of different size of dissolved gas and solvent molecules, but because of their different volatility. Another example of prior art in this area is described in the U.S. Pat. No. 5,263,409, where a membrane contactor was used to facilitate a contact between bittering agents present in a citrus juice and a hydrophobic extraction fluid. Extraction of organic components from aqueous solution into organic phase in a hollow fiber membrane module was described in J. of Membrane Science 50, 153-175, 1990